1. Field of the Invention
The present invention relates to a fire-resistant hydraulic fluid. More particularly, the invention relates to fire-resistant hydraulic fluid comprising the combination of a specific phosphoric acid triester and a specific polyester.
2. Description of Prior Arts
Fire-resistant hydraulic fluid of the phosphoric acid ester type has heretofore been commonly used as a fire-resistant hydraulic fluid, and it is composed mainly of a mixture of triaryl esters of phosphoric acid, the aryl group of which is phenol and lower alkyl-substituted phenols such as cresol, ethylphenol, dimethylphenol and isopropylphenol. A triaryl phosphate from phenol or cresol is excellent in the fire resistance, but the viscosity is so low that it cannot be used alone as a hydraulic fluid. Accordingly, such triaryl phosphate derived from phenol or cresol is used in combination with another triaryl phosphate such as derived from ethylphenol, dimethylphenol and isopropylphenol so as to have a desired viscosity suitable for a hydraulic fluid. However, these conventional hydraulic fluids have the following defects:
(1) The viscosity index is very low.
(2) The fire resistance is lower than that of a triester of phosphoric acid formed by using phenol or cresol alone.
(3) A tri-o-alkyl-substituted phenyl phosphate is inevitably included, whereby there is a likelihood of toxicity.
Among characteristic properties of a hydraulic fluid, the viscosity is very important. If the viscosity is too low, abrasion and leakage in a pump increase and the volumetric efficiency of the pump is reduced, resulting in the occurrence of seizure. When the viscosity is too high, the torque efficiency of the pump is lowered and so the pump does not work properly. It is said that a viscosity suitable for operation of, for example, a vane pump should be in the range from 20 to 400 centi-stokes. The viscosity of the fluid is greatly under influenced by environmental conditions, especially by the operating temperature. A fluid having a viscosity which may easily change depending on the operating conditions, especially temperature, that is, a fluid having a low viscosity index, can be used only in a narrow range of temperatures, and therefore the maintenance of the fluid is needed. Conventional fire-resistant hydraulic fluids of the phosphoric acid ester type are composed mainly of mixtures of triaryl phosphates derived from various alkyl-substituted phenols, and one of their defects is a low viscosity index.
It is known that in order to increase the viscosity in ordinary lubricating fluids, a thickener is added, and that a viscosity index improver is added in order to enhance the viscosity index. In the case of fire-resistant hydraulic fluids of the phosphoric acid ester type, however, there is observed a special phenomenon that the fire resistance is degraded by addition of a thickener such as a polyether or polyester.